This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Poly(A)-binding protein (PABP) is a critical regulatory protein that simultaneously controls mRNA translation and decay. PABP binds poly(A) RNA and organizes into undefined oligomerized structures along poly(A) tails that protect mRNA from nucleases in a repeating 27-nt pattern. PABP has four RRM motifs that bind RNA. A structure exists for RRM1-2 complexed with 7 adenylates. The C-terminal domain is enigmatic. It is proline and alanine-rich and nearly half the molecule. There is a small globular domain at the extreme C-terminus whose structure was solved by NMR, and contains a binding cleft for interactions with translation release factor 3(eRF3), and PABP regulatory protein (PAIP2), and poly(A)nuclease (Pan3). Other less defined regions of the C-terminal domain bind 60S ribosomes, initiation factors (eIF4B), translation stimulatory factors (Paip1) and ataxin2. Long term goals are to determine how interactions of PABP with eIF4G and eRF3 regulate de novo translation initiation versus ribosome 3'-5'recycling. Another long term goal is to understand how cleavage of select PABP moieties (directed by alternate conformations), is able to shut off poliovirus mRNA translation and simultaneously activate viral RNA replication on the same template. This is known to involve 5'-3 interactions between viral 3CD (RNA polymerase precursor), PCBP2 (cellular RNA-binding protein) on the viral 5'cloverleaf structure and PABP on the poly(A) tail. Viral RNA synthesis initiation occurs not at the 3'end of the poly(A) tail, but internally, 20 nucleotides from the 3'end of the 3'untranslated region (3). We hypothesize that one PABP is used as a molecular tool for polymerase placement on the template. However, there is no rationale for how one of 3-4 potential PABP moieties along the poly(A) tail is chosen as a molecular marker for directed cleavage and polymerase access to the template